[Greek phōs, phōt-, light.]
A photochemical action is caused by the chemical action of light, as for example in photography; a photoelectric process involves the emission of electrons from a surface by the action of light; a photophore (see -phore) is a light-producing organ in certain fishes and other animals; a photodetector responds to or detects incident light. A photon is a quantum of light or other electromagnetic radiation, and photonics is the branch of technology concerned with the properties and transmission of photons, for example in fibre optics.
In the second sense, relating to photography, the form derives from the first element of that word, which also appears in its own right, photo. Examples include photocall, an occasion on which famous people pose for photographers by arrangement; photojournalism, the art or practice of communicating news by photographs, especially in magazines; photomontage, a montage constructed from photographic images; photoreconnaissance, military reconnaissance carried out by means of aerial photography.